This invention relates generally to semiconductor device fabrication, and more particularly to the use of assist features in conjunction with off-axis illumination (OAI) for such fabrication.
Since the invention of the integrated circuit (IC), semiconductor chip features have become exponentially smaller and the number of transistors per device exponentially larger. Advanced IC""s with hundreds of millions of transistors at feature sizes of 0.25 micron, 0.18 micron, and less are becoming routine. Improvement in overlay tolerances in photolithography, and the introduction of new light sources with progressively shorter wavelengths, have allowed optical steppers to significantly reduce the resolution limit for semiconductor fabrication far beyond one micron. To continue to make chip features smaller, and increase the transistor density of semiconductor devices, IC""s have begun to be manufactured that have features smaller than the lithographic wavelength.
Sub-wavelength lithography, however, places large burdens on lithographic processes. Resolution of anything smaller than a wavelength is generally quite difficult. Pattern fidelity can deteriorate dramatically in sub-wavelength lithography. The resulting semiconductor features may deviate significantly in size and shape from the ideal pattern drawn by the circuit designer. These distortions include line-width variations dependent on pattern density, which affect a device""s speed of operation, and line-end shortening, which can break connections to contacts. To avoid these and other optical proximity effects, the semiconductor industry has attempted to compensate for them in the photomasks themselves, as well as by using other approaches.
This compensation in the masks themselves is generally referred to as optic al proximity correction (OPC). The goal of OPC is to produce smaller features in an IC using a given equipment set by enhancing the printability of a wafer pattern. OPC applies systematic changes to mask geometries to compensate for the nonlinear distortions caused by optical diffraction and resist process effects. A mask incorporating OPC is thus a system that negates undesirable distortion effects during pattern transfer. OPC works by making small changes to the IC layout that anticipate the distortions. OPC offers basic corrections and a useful amount of device yield improvement, and enables significant savings by extending the lifetime of existing lithography equipment. Distortions that can be corrected by OPC include line-end shortening, corner rounding, and isolated-dense proximity effect.
Isolated-dense proximity effect, or bias, in particular refers to the degree to which the mean of measured dense features differs from the mean of like-sized measured isolated features. Isolated-dense bias is especially important in the context of critical dimensions (CD""s), which are the geometries and spacings used to monitor the pattern size and ensure that it is within the customer""s specification. CD bias, therefore, refers to when the designed and actual values do not match. Ideally, bias approaches zero, but in actuality can measurably affect the resulting semiconductor device""s performance and operation. Isolated features, such as lines, can also negatively affect depth of focus (DOF), such that they cannot be focused as well with the lithography equipment as can dense features.
OPC can be used to correct the isolated-dense proximity effect and the isolated-feature DOF reduction by adding scattering bars (SB)""s) and anti-scattering bars (ASB)""s) near the edges of opaque and clear features, respectively, on a photomask. SB""s are sub-resolution opaque-like features, whereas ASB""s are sub-resolution clear-like features. SB""s and ASB""s are specific examples of assist features, which are features added to the mask that are not specifically part of the intended semiconductor design, but which assist the proper imprinting of the design on the photoresist. Both SB""s and ASB""s serve to alter the images of isolated and semi-isolated lines to match those of densely nested lines, and improve DOF so that isolated lines can be focused as well as dense lines can with the lithography equipment. For example, FIG. 1A shows a set of SB""s 100, whereas FIG. 1B shows the placement of such sets of SB""s 100 near an isolated line 102, in contradistinction to the dense sets of lines 104 and 106.
Another issue that impacts the quality of lithography is focus variation, which is nearly ubiquitous in IC manufacturing because of the combined effects of many problems, such as wafer non-flatness, auto-focus errors, leveling errors, lens heating, and so on. A useful lithographic process should be able to print acceptable patterns in the presence of focus variation. Similarly, a useful lithographic process should be able to print acceptable patterns in the presence of variation in the exposure dose, or energy, of the light source being used. To account for these simultaneous variations of exposure dose and focus, it is useful to map out the process window, such as an exposure-defocus (ED) window, within which acceptable lithographic quality occurs. The process window for a given feature, with or without OPC to compensate for distortions, shows the ranges of exposure dose and defocus that permit acceptable lithographic quality.
For example, FIG. 2 shows a graph 200 of a typical ED process window for a given semiconductor pattern feature. The y-axis 202 indicates exposure dose of the light source being used, whereas the x-axis 204 indicates defocus. The line 206 maps exposure dose versus defocus at one end of the tolerance range for the CD of the pattern feature, whereas the line 208 maps exposure dose versus defocus at the other end of the tolerance range for the CD of the feature. The area 210 enclosed by the lines 206 and 208 is the ED process window for the pattern feature, indicating the ranges of both defocus and exposure dose that permit acceptable lithographic quality of the feature. Any defocus-exposure dose pair that maps within the area 210 permits acceptable lithographic quality of the pattern feature. As indicated by the area 210, the process window is typically indicated as a rectangle, but this is not always the case, nor is it necessary.
Besides OPC, another approach that can be used to improve patterning is off-axis illumination (OAI). OAI is the shifting of the direction of the exposure beam during lithography from perpendicular, which interrupts the interference pattern that causes standing waves in the underlying photoresist being patterned. OAI particularly has the ability to significantly improve both the resolution and DOF for a given optical lithographic technology. For dense features, especially those having line-to-space duty ratios on the order of 1:1 to 1:2, such improvements are straightforward. Performance improvements are realized when illumination is obliquely incident on a mask at an angle so that the zeroth and first diffraction orders are distributed on alternative sides of the optical axis.
Examples of OAI are shown in FIGS. 3A and 3B. In FIG. 3A, the original center of illumination 302 has an illumination mask 304 positioned thereover. When an off-axis light source is instead used for illumination through the mask 304, first diffraction orders 306a and 306b result. The OAI of FIG. 3A is referred to as conventional OAI because the illumination mask 304 has a standard disc shape. In FIG. 3B, the original center 302 has an illumination mask 310 positioned thereover, resulting in diffraction orders 312a and 312b. The OAI of FIG. 3B is referred to as annular OAI because the illumination mask 310 has a ring shape. Other types of OAI include dipole, quadrupole, and quasar, which vary from one another and from conventional and annular OAI based on the illumination mask shapes used in such types of OAI.
Unfortunately, OAI does not improve resolution or DOF for isolated features. This is because discrete diffraction orders do not exist for isolated features; rather, a continuous diffraction pattern is produced. Therefore, OAI is used in conjunction with assist features, such as SB""s and ASB""s, so that both dense features and isolated features have larger process windows for photolithography. The addition of assist features to the mask effectively allows the isolated features to mimic the dense features during lithographic processes.
Most assist features for isolated features are intended for semiconductor features that are lines. Lines are essentially one-dimensional features for which assist features can be more easily formulated and added to the mask. Other types of features, however, are more two-dimensional in nature, such as contacts, which makes creation of assist features for such features more difficult. Contacts are typically, but not necessarily, to substantially square semiconductor features. They generally allow external electrical connectivity to semiconductor devices. Whereas OAI can improve resolution and DOF for dense arrays and groupings of contacts, it is not as successful for isolated, and semi-dense random contacts, which are generally referred to herein as non-dense contacts. Isolated contacts can more generally appear either randomly or on an orderly or regular basis within a design. Semi-dense contacts are those that appear with a periodicity less than a given threshold.
Because OAI does not improve resolution and DOF for such non-dense contacts, assist features are usually added to them. Different types of assist features have been proposed and used for such contacts, such as those shown in FIGS. 4A, 4B, and 4C. In FIG. 4A, an octagon-shaped assist feature 404 is used to improve photolithography of the contact 402. However, the assist feature 404 of FIG. 4A does not have a clear definition for semi-dense contacts. Furthermore, mask manufacturing of such a feature is difficult because of the very small forty-five degree angle patterns involved in the feature. In FIG. 4B, a square-shaped assist feature 406 is used to improved photolithography of the contact 402. Whereas the assist feature 406 of FIG. 4B is easier to print than the feature 404 of FIG. 4A is, it does not provide as good an improvement in the resulting photolithography. In FIG. 4C, the assist feature having lines 408a, 408b, 408c, and 408d is used to improve photolithography of the contact 402. However, the definition of the assist feature of FIG. 4C is not very clear for random contacts.
Therefore, there is a need for an assist feature for isolated, and semi-dense random contacts that overcomes the problems associated with the assist features for such non-dense contacts in the prior art. Such an assist feature should have clear definition for all of isolated, and semi-dense random contacts. Mask manufacture for such an assist feature should also not be overly difficult. For instance, the assist feature should be easily programmable with existing design-rule check (DRC) tools. For these and other reasons, there is a need for the present invention.
The invention relates to an assist feature for isolated, and semi-dense random contacts, as may be present on a photomask used in photolithographic processes for semiconductor device fabrication. The assist feature can be used in conjunction with off-axis illumination (OAI) for such non-dense contacts, for improving the depth of focus (DOF), resolution, or both of photolithography for such contacts, such that the non-dense contact at least more closely and preferably substantially mimics a dense contact during OAI. A photomask of the invention includes a number of assist features situated near and around the non-dense contact.
The size of the assist features is smaller than the size of the non-dense contact, such as between twenty and ninety percent of the size of the contact. There may be between six and twelve such assist features, and preferably there are eight. The assist features may be situated in a symmetrical and/or regular pattern around the non-dense contact, where each feature is equally distant from the contact. The features may further be substantially identical to one another, having a shape at least substantially similar to the shape of the non-dense contact.
The invention provides for advantages not found within the prior art. The assist features of the invention provide for clear definition of isolated, semi-dense, and other types of non-dense random contacts. The assist features increase depth of focus (DOF) and resolution for such contacts, and also reduce mask error enhancement factor (MEF). Photomask manufacture of such assist features is also relatively easy, because the assist features are easily programmable with existing design-rule check (DRC) tools.
Other advantages, embodiments, and aspects of the invention will become apparent by reading the detailed description that follows, and by referencing the attached drawings.